The present invention is directed to an apparatus and process for deflecting a laser beam through a precise 90.degree. angle using a penta-prism which is rotated to sweep out a flat plane perpendicular to the input laser beam, and more particularly to a technique for easily insuring the proper alignment of the laser beam with the penta-prism assembly.
In U.S. Pat. No. 4,297,031, and in co-pending U.S. Patent Application Ser. No. 313,965, filed Oct. 22, 1981, now U.S. Pat. No. 4,382,680 both by Martin R. Hamar, the entire disclosures of which are hereby incorporated by reference, a technique for sweeping a flat plane perpendicular to an input laser beam is disclosed. As noted therein, a penta-prism is mounted on a spindle and is aligned with an input laser beam. The input laser beam is reflected by the penta-prism to produce an output laser beam at right angles to the input laser beam. As the spindle is rotated, the output laser beam sweeps a plane perpendicular to the input laser beam.
When perfect conditions exists, namely when the axis of rotation of the penta-prism is aligned with the input laser beam, the output beam will sweep a perfectly flat plane. However, if the axis of rotation of the penta-prism is not aligned with the input laser beam, then optical and geometric translation errors in the output beam will occur, as described in more detail in the above-mentioned patent and co-pending patent application. Perfect alignment of the rotation and laser beam axes is very difficult to achieve.
In order to overcome the difficulties in aligning the rotation and laser beam axes while still producing a substantially flat plane perpendicular to the input laser beam, a technique is employed in the above patent and patent application which causes the optical and geometric errors produced by the lack of alignment to cancel each other out. Specifically, the point of intersection of the laser beam with the first reflecting surface of the prism is positioned approximatel a predetermined distance R from the point of intersection of the laser beam and penta-prism rotation axis. When this arrangement is achieved, the geometric and optical translation errors are equal in magnitude but opposite in sign and hence cancel to produce a truly flat plane.
As more fully described in the co-pending patent application, the predetermined distance R is given by R=D[1.707-3.414(1-1/N,)] when a solid penta-prism is employed, and R=1.707D when a hollow penta-prism is employed, where D is the distance the laser beam traverses within the penta-prism between the first and second reflecting surfaces, and N' is the index of refraction of the penta-prism.
When practicing the above described technique, it is still necessary to relatively position the penta-prism and laser such that the laser beam intersects the penta-prism rotation axis at or near the predetermined point. This is made difficult in some cases due to human error, machinery vibration, air turbulence, acoustic deflection, inter alia.